Addressing reporters in Dublin on Thursday afternoon, Mr Varadkar said Brexit "was a problem created in the UK" and it was up to Westminster to offer a solution.

"It was an opportunity to brain storm a bit as to what we could do to assist Prime Minister May in securing ratification of the withdrawal agreement," Mr Varadkar said of his conversation with Mrs Merkel.

'Red line'

Planning for a no-deal Brexit was also discussed during their phone call.

"What we both really agreed was that, once again, this is a problem that's created in London," Mr Varadkar said.

"The inability to ratify the withdrawal agreement is a problem in Westminster, and we're really looking to them for a solution," he said.

"But it has to be a proposal that we can accept. So it can't be a proposal that contradicts what is already in the withdrawal agreement.

"It can't be something that renders the backstop inoperable, for example. So we want to be in a position to give guarantees, give assurances, give clarifications."

The taoiseach added that the Irish border was the only "red line" his country has had in the Brexit negotiations and that would not change.

'East-west barrier'

The DUP criticised the Irish government's strategy after several of its senior figures had lunch with Mrs May on Thursday.

"The Irish Republic's 'no deal' preparations published just before Christmas have laid bare the nonsense propaganda about a hard border," Mr Dodds said.

"No one wants a hard border between Northern Ireland and the Republic. Indeed, its becoming clearer by the day that no one is ever going to construct such a border.

"With this clarity emerging in London, Dublin and Brussels, there is evidently no need for the aspects of the Withdrawal Agreement which have been so vigorously opposed by a broad cross section of the House of Commons."

Mr Dodds repeated his party's claim that the backstop would place a "barrier" between Northern Ireland and its "main trading partner", Great Britain.

"Brussels must now demonstrate that if it truly cares about Northern Ireland, then erecting a new east-west barrier should be no more palatable than having any new north-south barriers," he said.